1. Field of the Invention
The present invention relates generally to automotive lubrication system and more specifically to an oil pump which is suitable for use therein.
2. Description of the Prior Art
FIG. 1 shows a prior art trochoid type oil pump of the nature disclosed in Utility Model Publication JUM-A-59-88288. In this arrangement a pump casing 1 is formed with crescent shaped induction and discharge openings 2 and 3 respectively. An inner rotor 5 is mounted on an eccentric drive shaft 4 for synchronous rotation therewith and disposed within a ring shaped outer rotor 6.
In this arrangement the inner rotor is formed with 4 "external" teeth 7 while the outer rotor is formed with 5 "internal" teeth 8. With this arrangement when the drive shaft 4 is rotated by a non-illustrated connection with a prime mover such as an internal combustion engine, the inner and outer rotors rotate in unison. The inner rotor 4 moves within the outer rotor 6 in a manner to define spaces 9 into which oil from the induction opening 2 can enter and be retained in as they pass of the same. As the rotation of the rotors continues the spaces 9 are sequentially moved toward the discharge opening 3 and the oil which is inducted is subsequently compressed and squeezed out therethrough.
However, this arrangement has suffered from the drawback that during the rotation of the teeth of the inner and outer rotors come into mutual contact with one another and especially in the region of the discharge opening 3. Further, as each of the spaces 9 are isolated from one another some of the oil enclosed therein tends to get trapped and as the pulsation of the pump is extremely large, resonance noise tends to be generated.
In a second prior art arrangement of the nature disclosed in JP-A-57-79290 the oil pump has been constructed so that the teeth on the inner and outer rotors have asymmetrical profiles and wherein the contact ratio is less than 1. However, with this arrangement the curvature of the profile, that is to say, the radius of curvature of the faces and the top land portions of the teeth are extremely limited and machining of the the same requires a large number of intricate operations and precision machining. Even then the contact ratio of the internal and external teeth is less than one and in response to minor changes in rotation of the outer rotor the generation of relatively loud chattering noise is induced.